1. Field of the Invention
This invention relates to an optical type encoder, and in particular to an optical type encoder for illuminating a movable diffraction grating by a light beam, and detecting a phase variation in a diffracted light beam created by the movable diffraction grating which responds to the displacement of the movable diffraction grating, to thereby measure the displacement of the movable diffraction grating.
2. Related Background Art
In recent years, a precise displacement measuring machine capable of measuring the displacement of a movable body in the unit of 1 .mu.m or less (submicron) has been required in a precision instrument such as an NC machine tool or a semiconductor printing apparatus. As such measuring machine capable of measuring the displacement in the unit of submicron, there is well known a rotary encoder or a linear encoder which uses a coherent light beam such as a laser beam to form an interference fringe from a diffracted light beam from a movable body, and effects displacement measurement by a signal obtained by photoelectrically converting the interference fringe.
FIG. 1 of the accompanying drawings shows the construction of an example of the prior-art linear encoder of this type. In FIG. 1, the reference numeral 1 designates a laser, the reference numeral 2 denotes a collimator lens, and the reference numeral 3 designates a diffraction grating of grating pitch p mounted on a moving body, not shown, and moving at a velocity v, for example, in the direction of arrow. The reference numeral 51 and 52 denote quarter wavelength plates, the reference numerals 41 and 42 designate dach-prisms or corner cube reflecting mirrors, the reference numeral 6 denotes a beam splitter, and the reference numerals 71 and 72 designate polarizing plates whose polarization axes are orthogonal to each other and are disposed so as to form an angle of 45.degree. with respect to the polarization axes of the quarter wavelength plates 51 and 52. Numerals 81 and 82 respectively denote a light receiving element.
In FIG. 1, the light beam from the laser 1 is made into a substantially parallel light beam by the collimator lens 2 and enters the diffraction grating 3. Positive and negative mth-order diffracted light beams diffracted by the diffraction grating 3 again enters the diffraction grating 3 through the quarter wavelength plates 51 and 52, respectively, and through the corner cube reflecting mirrors 41 and 42, respectively, and become positive and negative mth-order re-diffracted light beams and overlap each other, and this overlapping light is divided into two light beams by the beam splitter 6, and the divided two light beams enter the light receiving elements 81 and 82 through the polarizing plates 71 and 72.
The so-called interferenced light beams entering the light receiving elements 81 and 82 are given a phase difference of 90.degree. therebetween by a combination of the quarter wavelength plates 51, 52 and the polarizing plates 71, 72, and are used for the discrimination of the direction of movement of the diffraction grating 3. The pulse number of a signal obtained by photoelectrically converting the interference fringe received by the light receiving elements 81 and 82 is counted to thereby find the amount of movement of the diffraction grating 3.
Numerous such optical type encoders utilizing the interference of diffracted light beams are shown, for example, in U.S. Pat. No. 3,726,595, 3,738,753, 3,756,723, 3,891,321, 4,629,886, 4,676,645, Japanese Laid-Open Patent Applications Nos. 58-191906, 58-191907, 60-190812, 61-65165, 61-178613, 61-212728, 62-6119 and 62-12814. However, these prior-art optical type encoders have tended to become complicated in construction if an attempt is made to enhance their resolving power and therefore, the applicant has proposed an optical type encoder which is simple and has a high resolving power in U.S. application Ser. No. 204,727, now U.S. Pat. No. 4,930,895.